Wash liquid production by heat exchange with low pressure liquid oxygen



March 5, 1968 $E|DEL 3,371,496

WASH LIQUID PRODUCTION BY HEAT EXCHANGE WITH LOW PRESSURE LIQUID OXYGEN Filed July 27, 1964 3 Sheets-Sheet 1 /n ve'mor MAX 5E I DE L I WWW Alfomey March 5, 1968 sElDEL 3,371,496

WASH LIQUID PRODUCTION BY HEAT EXCHANGE WITH LOW PRESSURE LIQUID OXYGEN Filed July 27, 1964 3 Sheets-Sheet 2 Fig. 2

/n van for MAX 55/051.

Ahomeys M. SEIDEL March 5, 1968 WASH LIQUID PRODUCTION BY HEAT EXCHANGE WITH Filed July 27; 1964 LOW PRESSURE LIQUID OXYGEN 3 Sheets-Sheet 5 lnvemor MAX SE/DEL United States Patent WASH LIQUID PRGDUCTION BY HEAT EX- CHANGE WITH LOW PRESSURE LIQUID OXYGEN Max Seidel, Munich-Salim, Germany, assignor to Linda Aktieugesellschar't, Wiesbatlen, Germany Filed July 27, 1964, Ser. No. 385,426 24 Claims. (Cl. 62-13) The present invention relates to the production of gases containing high concentrations of oxygen. More particularly, it relates to the production of gaseous mixtures containing from 70 to 98% of elemental oxygen by the fractionation of air.

A method for the production of oxygen gas of 70% purity has previously been described in German Patent No. 1,135,935 which discloses low temperature fractionation of air at elevated pressures in place of the customary two-stage process. In the process disclosed in the above patent oxygen enriched to 70% concentration is removed from the sump of a low pressure rectification column in the liquid state, and passed outside of the low pressure column in'heat exchange with the partially condensing oxygen-containing air fraction from the prewashing column for the air to be fractionated, the 70% oxygen being thereby vaporized. The condensate formed is then separated from the non-condensed portion of the oxygennitrogen mixture and is returned to the prewashing column for prewashing the air, while the non-condensed portion is delivered to the foot of the high pressure section (hereinafter designated alternatively as pressure column) of the double rectification column. The washing fluid which has been enriched to about 43% oxygen content in the prewashing column, after being cleaned in adsorbers, is delivered to the low pressure rectification column below the outlet of the oxygen-rich mixture from the foot of the pressure column.

The above process, however, has the disadvantage of a reduction in the economy of energy consumption as the oxygen content of the end product increases, as compared to the amount of energy required in the normal two column rectification process.

It is also known that a small amount of kryptonand xenon-rich concentrates can be withdrawn from the low pressure column and further evaporated to obtain krypton and xenon. The main portion of the required oxygen is then taken from one or more trays above the sump of the column without further enrichment. This procedure, however, does not in any manner reduce the energy consumption of the economy of the process.

It is an object of the present invention to modify the above procedure in such a manner that oxygen in a higher state of purity up to 98% and without further reduction of the energy consumption can be produced.

The above improvement is accomplished by withdrawing from the sump of the low pressure column a preliminary product containing a lower oxygen content than the end product, this preliminary product before its evaporation being brought to the oxygen content of the end product by intermediate evaporation with partial condensation of a further fraction passing from the prewashing column to a pressure column whereby the vapors obtained are returned to the foot of the low pressure column.

Another feature of the present invention resides in the fact that the intermediate concentration can be increased by enriching with oxygen the liquid preliminary product taken from'the low pressure column, this enrichment being effected by rectification by means of vapors formed during the concentration before the preliminary product is subjected to concentration.

The heaters used for the-intermediate concentration ice and for the vaporization of the end product are connected the head of the pressure column is condensed in a known manner. The condensation pressure of this nitrogen depends on the pressure in the sump of the low pressure column and on the oxygen content of the liquid to be vaporized. The liquid in the sump of the low pressure column has the same oxygen content as the preliminary product taken for concentration.

With a lower oxygen content of the vaporizing liquid there will be a lower boiling point. For enrichment of a lower oxygen content liquid, less rectification path and therefore less column resistance will be required. A lower degree of oxygen purity will be associated with a lower pressure in the sump of the column, leading to a further lowering of the boiling temperature of the vaporizing liquid. At the lower boiling temperature of the liquid to be vaporized in the sump of the low pressure column, an equally lower condensation temperature of the nitrogen from the head of the pressure column and therewith a correspondingly lower condensation pressure will suffice for heating the column.

The boiling temperature at which the intermediate product is concentrated and at which the end product is vaporized is higher, because of the higher oxygen content, than the boiling temperature in the sump of the low pressure column. Concentration of the intermediate product is continued until the liquid has an oxygen content equal to the oxygen content of the end product and under a pressure which, in case of the presence of an enriching rectification, is greater than the pressure over the sumpof the low pressure column by an amount equal to the resistance of the enriching rectification between the low pressure column and the intermediate vaporizer,

From that it follows that the boiling temperature during.

the intermediate concentration is higher than the boiling temperature of the liquid in the sump of the low pressure rectification-column.

In order to operate satisfactorily during the heating with the same fractionating pressure and with a condensation pressure which is greater only by the pressure resistance of the pressure column, an air. fraction having a low 0 content must be condensed and, in fact, according to the present invention, is in the form of the air to be fractionated which is being passed through the prewashing column. In the concentrating evaporator the evaporating liquid acquires an oxygen content which will be in equilibrium with the vapors of the end product, eg in the following ratios:

about 94% oxygen about 97% oxygen about 99% oxygen oxygen in gaseous end product: oxygen in gaseous end product: 98% oxygen in gaseous end product:

Oxygen content of the end product: Boiling temperature c'a. 90% oxygen T T =ca. 90% oxygen T -=T ca. 90% oxygen T T Accordingly, when the end product contains 90% oxygen the condensation sides of the intermediate vaporizer and of the end product vaporizer are arranged in parallel.

When the oxygen content of the end product is above about 90%, the air fraction coming from the prewashing column is first used to provide heat for the vaporization of the intermediate product and then, after separation of the oxygen-rich partial condensate, to effect vaporization of the end product by means of the remaining oxygen-poor vapors.

When the oxygen content of the end products is below about 90%, then on account of the higher boiling temperature during the end product vaporization, the heating is effected in the reverse order, first by vaporization of the end product and then with the remaining somewhat oxygen poor vapors by evaporation of the intermediate product. By complete or partial return of the partial condensate that results from the use of heat for vaporization of the intermediate and end products in preparation for pre-fractionation, the oxygen content of the air fraction that is used for heating can be influenced. The partial condensates in this case which are not returned to the fractionation column are returned to the pressure column together with the non-condensed portion of the air fraction used for heating.

In all cases the oxygen enrichment of the liquid drawn from the bottom of the pressure column and delivered to the low pressure column can be increased if a portion of the air fraction coming from the prewashing column is diverted from between the end product vaporizer and the intermediate vaporizer, or from ahead of both of the latter and after by-passing one or both Vaporizers, is introduced into the pressure column at a point below the inlet for the residual gases that were oxygen impoverished by partial condensation in the intermediate and final Vaporizers heated thereby. By the higher oxygen enrichment of the discharge from the pressure column, the rectification in the low pressure column will be relieved. There will not only be a diminution of the amount of oxygen-rich liquid taken from the foot of the prewashing column without proportionately increasing the amount of liquid taken from the foot of the pressure column, but less wash nitrogen will need to be taken from the upper portion of the pressure column in order to wash out the oxygen from the diminished amount of vapor in the column. As a result, there will remain a greater amount of nitrogen for the work-producing expansion for cold production, as well as an increased oXygen yield.

FIGURES l, 2 and 3 show schematically three specific examples of systems operating in accordance with the present invention for obtaining oxygen of 80%, 90% and 98% purity. Those elements which are the same in the figures are designated by the same reference characters. In order to illustrate more clearly the novel features of the invention for the production of oxygen of difierent degrees of purity, minor details which are not critical to the invention have been omitted. It is understood, therefore, that minor variations from the procedures set forth in the specific examples below which would be obvious to one skilled in the art are regarded as a part of the present invention, so long as they do not depart from the basic concept of the invention disclosed herein.

4 Production of ca. oxygen (FIGURE 1) Through conduit 1 were introduced 102,700 Nm. /h. of air under a pressure of approximately 4.0 atm. The air was cooled and freed from water vapor and carbon dioxide by passage first through regenerators 2, 4 and 6, and during the next switching period, through regenerators 3, 5 and 7. The remaining air, amounting to about 100,000 N /h., then passed through conduit 8 to the prewashing column 9 provided with rectification trays 10. In this column the remaining air was washed by the condensate produced during the heating of the final vaporizer 20 and intermediate vaporizer The washing liquid, amounting to about 35,000 Nm. /h., and the composition of which was in equilibrium with that of the entering gaseous air and whose oxygen content was about 43%, was drained from the foot of this column by conduit 11 for delivery to one of the transposable silica gel adsorbers 12a, 12b where it was freed from the residual impurities, such as carbon dioxide and hydrocarbons, which were washed out from the entire volume of entering air.

The cleansed liquid was then passed by conduit 13 through heat exchanger 14 and expansion valve 17 to the inlet 18 of low pressure column 16 where it was expanded to the pressure of about 1.3 atm. which prevailed in the column.

The cleansed gaseous air fraction flowing from the head of the prewashing column 9 had an oxygen content of about 14%. A portion or this air fraction was delivered by conduit 19a to the final vaporizer 20 in which it was partially liquefied by heat exchange with the liquefied end product delivered by conduit 21 from rectification column 102, the partially liquefied air fraction being there delivered to the separator 22.

The other parallel portion of this air fraction of the prewashing column 9 was delivered by conduit 19b to the intermediate vaporizer 100 in which it was partially liquefied by heat exchange with the partially vaporized liquid product delivered by conduit 53 from the rectification column 102, the partially liquefied portion being then delivered to the separator 101. In the separators 22 and 101 the liquefied portions were separated and delivered collectively through conduit 23 as a washing liquid containing about 21% oxygen to the head of prewashing column 9 while the gaseous portions amounting to about 65,000 Nrnfi/ h. with an oxygen content of about 9% were passed by conduits 24a and 24b from the heads of the separators and delivered collectively by conduit 24 to pressure column 15 of the double rectifier which was under a pressure of about 3.7 atm. The liquid which collected in the sump of this column, amounting to about 21,000 Nmfi/h. with an oxygen content of about 21%, was removed by conduit 25, passed through heat exchanger 26 to be further cooled, and was then passed through expansion valve 27 for delivery to the low pressure column 16 at a point 28 above the point 18 where the liquid from the prewashing column 9 was introduced. From the upper portion of the pressure column 15 about 20,400 Nm. /h. of liquid nitrogen were removed by conduit 29, further cooled in heat exchanger 30 and after passage through expansion valve 31 delivered by conduit 32 as a washing liquid into the upper portion of the low pressure column 16. Gaseous nitrogen was removed from the head of the pressure column by conduit 33 and warmed somewhat in the heat exchanger 34.

About 23,500 Nm. /h. of the above nitrogen were then delivered by conduit 35 to turbine 36 to be expanded therein while doing work. This nitrogen together with about 54,500 Nm. /h. nitrogen containing about 2% of oxygen, which was withdrawn by conduit 38 from the head of the low pressure column 16 and then warmed in heat exchangers 30 and 26, was then passed through heat exchanger 14. About 78,000 Nm. /l1. of nitrogen were then passed through the conduit 37 to regenerators 3 and 5 or 2 and 4 to be warmed therein to room temperature and were then conducted away by conduit 39.

A portion of the nitrogen which had been somewhat warmed in the heat exchanger 34 was delivered through conduit 40 to the heat exchanger 41 where it was warmed to room temperature countercurrently to itself while being compressed in a dry run compresser 42 to about atm. and then again cooled in heat exchanger 41 and liquefied in heat exchanger 34. The liquid nitrogen was then passed through expansion valve 43 and through valve 445 to the high pressure column 15, and/ or was added via valve 44a to the liquid nitrogen flowing from the high pressure column 15 through heat exchanger 30 to the low pressure column 16.

During cold running of the apparatus the nitrogen in heat exchanger 34 and expanded in valve 43 was passed through valve 44b and through separator 101 for delivery to the prewashing column 9 in order to prewash the air prior to the separation of the liquefied end products.

In the sump of low pressure column 16 liquefied oxygen of about 80% purity collected as a preliminary product. It was then delivered by conduit 52 to the enrichment rectification column 102 as return flow liquid. The drained preliminary product now further enriched with oxygen was delivered by conduit 53 from the foot of this column to the intermediate vaporizer 100 to be further enriched to an end product purity of about 90%.

The vapors produced in the intermediate vaporizer 100 were then delivered to the enrichment rectification column 102 in mass transfer with the outward flowing preliminary product, while gaseous residue containing about 53% oxygen taken from the head of this column was delivered by conduit 54 to the low pressure column 16.

The intermediate vaporizer 100 is represented as a circulating vaporizer. The portion of the vaporizing liquid remaining after circulation was returned by conduit 53 to the preliminary product in the intermediate vaporizer. The 90% oxygen liquid end product was delivered from the rectification column 102 through conduit 21 to the final vaporizer in which it was vaporized. In the associated separator 45 the liquid portion remaining after circulation of the vaporizing liquid, having an oxygen content of about 97% in equilibrium with the vapors containing 90% oxygen, was separated and removed from the foot of the separator by conduit 46 for passage through the adsorber 47 which during continuous circulation was traversed by the vaporizing liquid. The gaseous end product, about 22,000 Nm. /h. 90% oxygen, consisting of 19,200 Nmfi/h. of pure oxygen and 2,800 Nm. /h. of air, was removed by conduit 48 from the head of the separator 45, to be warmed in the regenerator 7 to room temperature from which is escaped through conduit 49.

The functioning of the regenerator pairs 2/3, 4/5 and 6/7 was reversed periodically in a known manner. The settings of the valve groups 50a and 50c to the warm sides of the regenerators and of the groups of recoil valves 51a and 51c to the cold sides are indicated by the flow directions in the regenerators.

In the possible variants of this system which have already been mentioned, it is possible under some conditions to omit the nitrogen circulation through heat exchangers 34 and 41, compressor 42 and expansion valve 43 together with the conduits associated therewith.

In this case the compressed nitrogen to be passed through the expansion turbine 36 can be prewarmed in heating coils in the regenerator pairs, or a pair of regenerators is provided With a central pipe connection from which a portion of the air to be fractionated after being passed through one of the pair of transposable adsorbers is passed through an expansion turbine and then into the low pressure column.

Production of 80% oxygen (FIGURE 2) From an initial 102,500 Nm. /h. of air at a pressure of 3.6 atm., 100,000 Nm. /h. remained for delivery to the prewashing column 9 after the losses resulting from periodic switching. The air fraction taken from the head of the prewashing column 9 with an oxygen content of about 17% was partly liquefied in the final vaporizer 20 and the liquid portion containing about 28% oxygen returned to the head of the prewashing column 9. The washing liquid which drained from the prewashing column, amounting to about 29,000 Nmfi/ h. containing 43% oxygen, was conducted to the low pressure column in which the pressure was 1.3 atm. From the non-condensed residue of about 71,000 Nm. /h. containing about 12% oxygen in the final vaporizer, 31,000 Nm. /h. were delivered to the lower end of the pressure column 15 maintained at a pressure of about 3.2 atm. The remaining 40,000 Nm. /h. were partially condensed in the intermediate vaporizer 100 and the gas-liquid mixture containing about 9% oxygen in the gaseous portion and about 22% in the liquid portion was delivered to the pressure column 15 at a suitable point about the place Where the non-condensable portion from the final vaporizer 20 is received. From the head of the pressure column 15 about 27,200 Nmfi/h. of gaseous nitrogen were removed for the ex pansion turbine 36. From the upper portion of the pressure column 15 about 16,500 Nm. /h. of liquid nitrogen were delivered to the low pressure column 16 as washing liquid. From the head of the low pressure column'16 about 48,000 Nm. h. of nitrogen still containing an oxygen content of about 1% was removed.

The liquid collecting in the sump of the low pressure column 16, containing about 70% of oxygen, was enriched to about in the enriching rectifier column 102 and in the intermediate vaporizer and was then delivered to the final vaporizer 20. The gaseous end product was composed of 25,200 Nm. /l1. of 80% oxygen, consisting of 18,800 Nm. /h. of pure oxygen and 6,400 Nm. /h. of air.

Production 0 98% oxygen (FIGURE 3) From 103,000 Nm. /h. of air under a pressure of about 4.1 atm. about 100,000 Nm. /h. were delivered to the preheating column 9 after the losses caused by switching. From the air fraction having an oxygen content of about 20.6%, taken from the head of the prewashing column, about 31,200 Nm. /h. were delivered directly to the lower end of the pressure column 15 maintained under a pressure of 3.8 atm. and 71,300 Nm. /h. partially liquefied in the intermediate vaporizer 100. The liquid portion containing about 36.5% of oxygen, was returned to the head of the prewashing column. The washing liquid was drained from the prewashing column, amounting to about 11,000 Nm. /h. containing about 43% of oxygen, was delivered to the low pressure column 16 in which the pressure was 1.3 atm. The portion which was uncondensed in the intermediate vaporizer, namely 57,500 Nm. /h. containing about 17% of oxygen, was partially liquefied in the final vaporizer 20, and the gasliquid mixture containing about 10.5% of oxygen in the gas and about 25% of oxygen in the liquid was delivered to the pressure column 15 at a suitable point about the place where the partial current from the head of the prewashing column was introduced. From the head of the pressure column 22,300 Nm. h. of gaseous nitrogen were removed for delivery to the expansion turbine 36. About 22,800 Nm. h. of liquid nitrogen containing about 9% of oxygen were delivered from the upper portion of the pressure column as washing liquid into the low pressure column 16. From the sum of the pressure column 15 about 43,600 Nrn. /h. of product containing about 33% of oxygen were removed for delivery to the low pressure column 16. From the head of the low pressure column about 58,700 Nmfi/h. of nitrogen still containing about 3.5% of oxygen were withdrawn. The 82% of oxygen which collected in the sump of the low pressure column was enriched to about 98% oxygen content in the enrichment rectification column 102 and in the intermediate vaporizer 100, and then delivered to the final vaporizer 20. The gaseous end product was composed of about 19,000 Nm. /h, of 98% oxygen, consisting of about 18,600 Nm. /h. of pure oxygen and about 400 Nm. /h. of air.

What is claimed is:

1. A process for producing gas of 70-98% oxygen purity in a double rectifier having a high pressure column and a low pressure column, comprising the steps of:

(a) conducting compressed air to be fractionated through regenerators in indirect heat exchange with fractionation products;

(b) conducting cooled gaseous air to a prewashing column and washing the air with liquid air enriched in oxygen;

() withdrawing liquid preliminary product from the sump of the low pressure column to an intermediate partial vaporization zone and partially vaporizing said preliminary product into a first liquid fraction and a first vapor fraction;

(d) recycling said first vapor fraction to the bottom portion of the low pressure column;

(e) conducting said first liquid fraction to a final partial vaporization zone separate and distinct from said intermediate partial vaporization zone to partially vaporize said first liquid fraction to obtain a second liquid fraction and a second vapor fraction;

(f) conducting partial vaporization of at least one of steps (c) and (e) in indirect heat exchange with washed cold gaseous air from step (b) to partially condense same;

(g) recycling resultant liquid from partially condensed air as washing liquid to said prewashing column; and

(h) conducting resultant loaded Washing liquid to the low pressure column.

2. The process of claim 1 wherein the liquid preliminary product from the sump of the low pressure column is enriched with oxygen by rectification by means of the first vapor fraction generated by the intermediate partial vaporization, before said preliminary product is subjected to intermediate partial vaporization.

3. The process of claim 2 wherein a portion of said resultant liquid from partially condensed air, step (g), instead of being delivered to the prewashing column, is delivered to the lower portion of the high pressure column at a point corresponding to its composition.

4. The process of claim 1 wherein one portion of the washed cold gaseous air from the prewashing column, step (b), is passed directly to the foot of the high pressure column and another portion is used to furnish heat for the intermediate partial vaporization, and for the final partial vaporization of said second liquid fraction, and is then passed to the high pressure column at a point above that at which the first portion is introduced.

5. The process of claim 4 wherein resultant condensed air formed during the intermediate vaporization is separated from the non-condensed portion of the fractionated air and delivered as a washing liquid to the prewashing column.

6. The process of claim 1 wherein the liquid preliminary product from the sump of the low pressure column is enriched with oxygen by rectification by means of the first vapor fraction generated by the intermediate partial vaporization, before said preliminary product is subjected to intermediate partial vaporization, and one portion of the washed cold gaseous air from the prewashing column, step (b), is passed directly to the foot of the pressure column and another portion is used to furnish heat for the intermediate partial vaporization and for the final vaporization of the product, and is then passed to the high pressure column at a point above that at which the first portion is introduced.

7. The process of claim 1 wherein step (f) is conducted by conducting partial vaporization of step (c), and resultant condensed air formed during the intermediate partial vaporization is separated from the non-condensed portion of the fractionated air and delivered as a washing liquid to the prewashing column.

8. The process of claim 7 wherein the liquid preliminary product from the sump of the low pressure column is enriched with oxygen by rectification by means of the first vapor fraction generated by the intermediate partial vaporization, before said preliminary product is subjected to intermediate partial vaporization.

9. The process of claim 1 wherein a portion of said resultant liquid from partially condensed air, step (g), instead of being delivered to the prewashing column, is delivered to the lower portion of the high pressure column at a point corresponding to its composition.

10. The process of claim 1 wherein for the production of oxygen of approximately purity, the intermediate partial vaporization and the final partial vaporization are performed with two parallel currents of the air fraction coming from the prewashing column.

11. The process of claim 1 wherein for the production of less than about 90% purity, the air fraction from the prewashing column is used first to provide heat for the final partial vaporization and then to provide heat for the intermediate partial vaporization, said heat in both partial vaporizations being derived from condensing air.

12. The process of claim 1 wherein for the production of oxygen of in excess of 90% purity, the air fraction from the prewashing column is used first to provide heat for the intermediate partial vaporization and after separation of the condensed air thus produced is used to provide heat for the final partial vaporization, said heat in both partial vaporizations being derived from condensing arr.

13. The process of claim 1 wherein a portion of the condensed air formed by the intermediate partial vaporization and final partial vaporization, instead of being delivered to the prewashing column, is delivered to the lower portion of the pressure column at a point corresponding to its composition, only the condensed air produced by the intermediate vaporization from the heat supplying air fraction being delivered as a washing liquid to the prewashing column, while the condensed air produced by the final partial vaporization is delivered to the high pressure column at a point of corresponding composition.

14. The process of claim 1 wherein a portion of the condensed air formed by the intermediate partial vaporization and final partial vaporization, instead of being delivered to the prewashing column, is delivered to the lower portion of the high pressure column at a point corresponding to its composition, only the condensed air resulting from the final partial vaporization being delivered to the prewashing column as a washing liquid, while the condensed air resulting from the intermediate partial vaporization is delivered to the high pressure column of the double rectifier at a point of corresponding composition.

15. The process of claim 1 wherein the intermediate partial vaporization is performed as a circulating vaporization wherein the non-vaporized liquid is separated during each circulation from its vapors, a portion of said liquid being further concentrated to produce said first liquid fraction which is to undergo said final partial vaporization, while the remainder of said liquid together with the preliminary product from the low pressure rectification column is admixed and subjected to further concentration.

16. An apparatus including a double rectifier in combination with regenerators for heat exchange between the air to be fractionated and the fractionation products and a final vaporizer for vaporization of the liquid products taken from the sump of the low pressure column by heat exchange with a partially condensing fraction taken in gaseous form from a prewashing column and its liquefied portion being returned as washing liquid to the prewashing column while the washing liquid which drains from the prewashing column is delivered to the low pressure column at a point below the point where the liquid from the sump of the high pressure column is introduced, the improvement which comprises an intermediate vaporizer distinct and separate from said final vaporizer and connected to the sump of the low pressure column to receive liquid therefrom to be concentrated while the vapors are returned to the column, means for heating said vaporizer by an air fraction taken from the head of a prewashing column, and means for returning at least a portion of said air fraction to the lower part of the said high pressure column.

17. The apparatus of claim 16 wherein an enrichment rectification column is connected in the liquid and vapor path between the vaporizing side of the intermediate vaporizer and the sump of the low pressure column.

18. The apparatus of claim 16 wherein the intermediate vaporizer is in the form of a circulating vaporizer with means for delivering its vapors to the low pressure column, in combination with means for delivering its liquid to the final vaporizer.

19. The apparatus of claim 16, further comprising condensate separators in conduits that receive the partially condensed air fractions from the intermediate vaporizer and the final vaporizer, the tops of the separators being connected to the lower portion of the high pressure column while their bottoms are connected to the head of the prewashing column.

20. The apparatus of claim 16 comprising a condensate separator in the discharge conduit for the partly condensed air fraction from the intermediate or final vaporizer, the upper part of the separator being connected to the lower part of the high pressure column while the lower part is connected to the head of the prewashing column, the outlet conduit for the partly condensed air fraction from one of the two Vaporizers being connected directly to the lower part of the high pressure column.

21. The apparatus of claim 16 comprising a conduit with a throttle valve directly between the head of the 1G prewashing column and the foot of the high pressure column, and a parallel conduit having in its path the heating jackets of the intermediate vaporizer and final vaporizer.

22. The apparatus of claim 16 comprising a conduit with a throttle valve directly between the head of the prewashing column and the foot of the high pressure column, and a parallel conduit having in its path the heating jackets of the intermediate vaporizer and final vaporizer, the heating jackets of the intermediate vaporizer and final vaporizer being connected in parallel.

23. The apparatus of claim 16, comprising a conduit with a throttle valve directly between the head of the prewashing column and the foot of the high pressure column, and a parallel conduit having in its path the heating jackets of the intermediate vaporizer and final vaporizer, having heating jackets of the intermediate vaporizer and final vaporizer connected in series for countercurrent stagewise vaporization.

24. The apparatus of claim 16 comprising a conduit with a throttle valve directly between the head of the prewashing column and the foot of the high pressure column, and a parallel conduit having in its path the heating jackets of the intermediate vaporizer and final vaporizer, connected in series for unidirectional stagewise vaporization.

References Cited UNITED STATES PATENTS 2,513,306 7/1950 Garbo 62-29 X 2,619,810 12/1952 Rice et al. 6214 2,664,719 1/1954 Rice et al 6214 X 2,812,645 11/1957 Locklair et al. 6214 2,873,583 2/1959 Potts et al 6214 FOREIGN PATENTS 1,135,935 9/1962 Germany NORMAN YUDKOFF, Primary Examiner.

W. PRETKA, Assistant Examiner. 

1. A PROCESS FOR PRODUCING GAS OF 70-98% OXYGEN PURITY IN AN DOUBLE RECTIFIER HAVING A HIGH PRESSURE COLUMN AND A LOW PRESSURE COLUMN, COMPRISING THE STEPS OF: (A) CONDUCTING COMPRESSED AIR TO BE FRACTIONATED THROUGH REGENERATORS IN INDIRECT HEAT EXCHANGE WITH FRACTIONATION PRODUCTS; (B) CONDUCTING COOLED GASEOUS AIR TO A PREWASHING COLUMN AND WASHING THE AIR WITH LIQUID AIR ENRICHED IN OXYGEN; (C) WITHDRAWING LIQUID PRELIMINARY PRODUCT FROM THE SUMP OF THE LOW PRESSURE COLUMN TO AN INTERMEDIATE PARTIAL VAPORIZATION ZONE AND PARTIALLY VAPORIZING SAID PRELIMINARY PRODUCT INTO A FIRST LIQUID FRACTION AND A FIRST VAPOR FRACTION; (D) RECYCLING SAID FIRST VAPOR FRACTION TO THE BOTTOM PORTION OF THE LOW PRESSURE COLUMN; (E) CONDUCTING SAID FIRST LIQUID FRACTION TO A FINAL PARTIAL VAPORIZATION ZONE SEPARATE AND DISTINCT FROM SAID INTERMEDIATE PARTIAL VAPORIZATION ZONE TO PARTIALLY VAPORIZE SAID FIRST LIQUID FRACTION TO OBTAIN A SECOND LIQUID FRACTION AND A SECOND VAPOR FRACTION; (F) CONDUCTING PARTIAL VAPORIZATION OF AT LEAST ONE OF STEPS (C) AND (E) IN INDIRECT HEAT EXCHANGE WITH WASHED COLD GASEOUS AIR FROM STEP (B) TO PARTIALLY CONDENSE SAME; (G) RECYCLING RESULTANT LIQUID FROM PARTIALLY CONDENSED AIR AS WASHING LIQUID TO SAID PREWASHING COLUMN; AND (H) CONDUCTING RESULTANT LOADED WASHING LIQUID TO THE LOW PRESSURE COLUMN. 